Until fairly recently, the preferred, indeed the only means by which to display information in the electronic medium was to use a video monitor comprising a cathode ray tube ("CRT"). CRT technology has been well known for over 50 years, and has gained widespread commercial acceptance in applications ranging from desktop computer modules to home televisions and industrial applications. CRTs are essentially large vacuum tubes having one substantially planar surface upon which information is displayed. Coated on the inside of the CRT planar surface is a layer of phosphors which respond by emitting light when struck by electrons emitted from the electron gun of the CRT. The electron gun is disposed in an elongated portion which extends away from the inside of the CRT display surface.
While CRTs are widely used in numerous applications, there are several inherent limitations to the application of CRT technology. For example, CRTs are relatively large and consume a great deal of energy. Moreover, as they are fabricated of glass, the larger the display surface, the heavier the CRT. Given the need for the electron gun to be spacedly disposed from the phosphorus surface of the display surface, CRTs have a substantial depth dimension. Accordingly, CRTs have little use in small and portable applications, such as handheld televisions, laptop computers, and other portable electronic applications which require the use of displays.
To answer the needs of the marketplace for smaller, more portable display devices, manufacturers have created numerous types of flat panel display devices. Examples of flat panel display devices include active matrix liquid crystal displays (AMLCD's), plasma displays, and electroluminescent displays. Each of these types of displays has use for a particular market application, though each are accompanied by various limitations which make them less than ideal for certain applications. Principal limitations inherent in devices such as AMLCD's relate to the fact that they are fabricated predominantly of inorganic semiconductor materials by semiconductor fabrication processes. These materials and processes are extremely expensive, and due to the complexity of the manufacturing process, cannot be reliably manufactured in high yields. Accordingly, the costs of these devices are very high with no promise of immediate cost reduction.
One preferred type of device which is currently receiving substantial research effort is the organic electroluminescent device. Organic electroluminescent devices ("OED") are generally composed of three layers of organic molecules sandwiched between transparent, conductive and/or metallic conductive electrodes. The three layers include an electron transporting layer, an emissive layer, and a hole transporting layer. Charge carriers specifically, electrons and holes, are generated in the electron and hole transporting region. Electrons are negatively charged atomic particles and holes are the positively charged counterparts. The charge carriers are injected into the emissive layer, where they combine, emitting light.
OED's are attractive owing to low driving voltage, i.e., less than about 20 volts. Hence, they have a potential application to full color flat emissive displays. OED's have heretofore suffered from relatively short usage lifetimes. Though significant lifetime for OED's has been achieved as in, for example, U.S. Pat. No. 4,720,432 to VanSlyke, et al, further improvement is needed, particularly in applications where high brightness is required. In general, the hole transporting materials used in the hole transporting layer of an OED are the most vulnerable parts, and hence, contribute significantly to shortened lifetime for devices. The hole transporting layers are typically susceptible to thermal degradation by physical aggregation or recrystallization.
To address the thermal degradation problems associated with hole transporting materials in OED's, hole transporting materials characterized by high glass transition temperatures, and bonded onto a polymer have been proposed. These are shown in, for example, U.S. Pat. No. 5,061,569 and 5,443,921. While the thermal degradation problem has been somewhat alleviated by the solution proposed in the aforementioned patents, they have nonetheless continued to plague the hole transporting materials used in OED's.
Another approach described in U.S. Pat. No. 5,518,824 to Funhoff, et al, discloses an electroluminescent arrangement containing one or more organic layers, at least one of the layers being obtained by thermal or radiation induced crosslinking, and where at least one charge transporting compound is present per layer. While the '824 patent appears to have achieved some improvement in thermal degradation, it has come at the cost of high operating voltage. Specifically, operating voltage of the device disclosed in the '824 patent is on the order of 93 volts, far too high for practical use in portable applications.
Accordingly, there exists a need for improved materials for use as the hole transporting layer in OED's. The material should be relatively inexpensive and easy to fabricate as well as being conducive to manufacturing in the current OED manufacturing process. The device should have good thermal stability, and be capable of operating at voltages which are within the range of those generally accepted for OED's.